HTML Guides for view-transition

The @view-transition at-rule is valid CSS but is not yet recognized by the W3C CSS validator because it is a relatively new feature defined in the CSS View Transitions Module Level 2 specification.

The @view-transition at-rule opts a document into cross-document view transitions when navigating between two same-origin pages. It is placed in the CSS of the destination page (the page being navigated to) and accepts a navigation descriptor that controls when the transition activates.

@view-transition{

navigation: auto;

}

The navigation descriptor accepts these values:

• none — no cross-document view transition occurs (default).
• auto — the transition activates for same-origin navigations where the navigation type is traverse, push, or replace, as long as the navigation does not include a cross-origin redirect.

Because the W3C CSS validator has not yet added support for this at-rule, the warning cannot be fixed by changing your code. The CSS itself is correct per the specification. Browser support is available in Chromium-based browsers (Chrome 126+, Edge 126+).

You can safely ignore this validator warning. If you want a completely clean validation output, move the @view-transition rule into a separate stylesheet so it does not interfere with validation of the rest of your CSS, or suppress the warning in your CI pipeline.